The cat has long been a standard animal for anatomical and acute physiological studies of muscle function and motor control at the spinal cord level. In this project, a wide variety of traditional and novel kinesiological techniques are being used to study motor tasks in unanesthetized, normally behaving cats, including computer-aided reconstruction of skeletal movement from videotape, multiaxis force plates, chronically implanted nerve cuff and EMG electrodes, and strain and length transducers. The major focus has been the study of hindlimb muscles and their afferent and efferent control during walking, which is the subject of a computer modeling project described in (Project No. Z01-NS-02079-13). Other hindlimb movements studied include jumping, paw shaking, scratching, and reflexes to cutaneous nerve stimulation during normal and decerebrate walking. In a collaborative study, similar data are being collected from a large number of neck muscles. The major objective is to correlate patterns of usage with complex mechanics and compartmentalization and proprioceptive specializations of these muscles. A major theme emerging from these experiments is a concept of "Task Groups," which denotes the segregation and specialization of sensorimotor systems to perform kinematically homogeneous tasks in an optimal manner. This is particularly apparent in multiarticular muscles, which in some cases use independent subdivisions of their alpha motoneuron pool to accomplish kinematically diverse tasks. Recent work has concentrated on functional architecture of muscle units, including mechanical arrangements of their constituent fibers, segregation on the basis of histochemical fiber types, and compartmentalized patterns of EMG recruitment during normal motor behaviors. We believe an overview of these various organizational features of many different motor pools will provide insights into both functional significance of these features and their implications for the spinal segmental circuitry that regulates and coordinates the various muscles.